Endogenous oxidative damage to nuclear DNA is an early event in ischemic brain injury that may trigger neuronal cell death. DNA base-excision-repair (BER), consisting of the short-patch and long-patch repair pathways, is the predominant repair mechanism of endogenous oxidative DNA damage in the brain. BER is an inducible mechanism; induced BER activity in ischemic brain has been associated with rapid repair of DNA damage and cell survival. Thus, we hypothesize that the inducible BER activity constitutes an endogenous mechanism of neuroprotection that determines, at least in part, the outcome of ischemic brain injury. Exciting and relevant preliminary data supporting this hypothesis have now been obtained. These include the finding that increased expression of BER enzymes and induced cellular BER activity via endogenous (ischemic preconditioning) or ekogenous (vector-mediated gene transfection) mechanisms enable neurons to be tolerant to subsequent ischemic injury and related insults. The overall objective of this application for competing renewal is to further explore the neuroprotective role of inducible BER activity in ischernic brain and in cell culture model of neuronal ischemia, and to elucidate the mechanism by which this important cellular mechanism is activated. The specific aims of this project are to: 1. Study the mechanism and functional role of inducible DNA base-excision-repair activity in a rat model of ischemic tolerance. 2. Study the mechanism and functional role of inducible DNA base-excision-repair activity in cell culture models of neuronal ischemia and tolerance. 3. Determine if inducible DNA base-excision-repair activity is neuroprotective by altering BER gene expression in vivo using adeno-associated virus expression vectors. Endogenous adaptive responses, such as tolerance, are evolutionarily highly conserved, and may reveal particularly relevant protective neurobiological mechanisms. Therefore, understanding the role of inducible BER activity in the mechanism underlying tolerance may yield novel insights into the mechanism by which the brain's endogenous protective capacity functions. Studies in both in vivo and in vitro models are proposed. The in vivo animal model of cerebral ischernia minics many aspects of pathophysiological changes in the brain after clinical ischemia. The in vitro models will complement the in vivo studies by allowing for precise mechanistic studies to be performed.